Thin film heating for the preliminary purification of crude phthalic anhydride

ABSTRACT

In the preliminary purification (prior to distillation) of crude phthalic anhydride produced from xylene, the pretreatment is improved by heating a thin film of molten phthalic anhydride to 200-280* C. during the course of not more than 60 seconds, said film having a thickness of not more than 2 mm.

United States Patent Gehrken et a1.

[ 51 Mar. 21, 1972 THIN FILM HEATING FOR THE PRELIMINARY PURIFICATION OFCRUDE PHTHALIC ANHYDRIDE Inventors: Hubert Gehrken, Weiden; Gerd Helms,.lunkersdorf; Gerhard Keunecke, Geyen; Herbert Krimphove, Cologne,Mungersdorf, all of Germany Chemiebau Dr. A. Zieren GmbI-I & Co. KG,Cologne-Mungersdorf, Germany Filed: Apr. 14, 1969 Appl. No.: 816,112

Assignee:

Foreign Application Priority Data [56] References Cited UNITED STATESPATENTS 3,011,955 12/1961 Brown ....260/346.7 X 3,199,574 8/1965 Keller..159/6 W 3,311,457 3/1967 Goosens ...159/6 W X 3,420,750 1/1969Schaefer et a1. ..203/89 X 3,428,423 2/1969 Egbert ...260/346.7 X

Primary ExaminerNorman Yudkoff Assistant Examiner-4. Sofer Attorney-I.William Millen, M. Ted Raptes and John L, White [57] ABSTRACT In thepreliminary purification (prior to distillation) of crude phthalicanhydride produced from xylene, the pretreatment is improved by heatinga thin film of molten phthalic anhydride to ZOO-280 C. during the courseof not more than 60 seconds, said film having a thickness of not morethan 2 mm.

17 Claims, 1 Drawing Figure VOLATILE lMPUR-ITIES PURE PA H 3 PHTHALIC II 220C ANHYDRIDE DISTILLATION STORAGE COLUMN FOR CRUDE 2/ 240 4 f '0 PAPRODUCED 245% FROM XYLENE ,l 215- 6 2aoc mm FILM EVAPORATOR /9 n STORAGEFOR RESIDUE PRETREATED I PAIENIEI] MR2] I972 9.0mm mmm uemwm nmw 5 @EEE55: me $365 m m 2 2:38 292 355 MQEQEE 2 25% m N. L E E2 P T 8:52;

INVENTORS HUBERT GEHRKEN GERD HELMS mass men @565 ATTORNEYS THIN FILMHEATING FOR THE PRELIMINARY PURIFICATION OF CRUDE PHTHALIC ANI-IYDRIDEBACKGROUND OF THE INVENTION This invention relates to a process for thecontinuous preliminary purification of crude phthalic anhydride producedfrom o-xylene, wherein said crude is heated to a treatment temperatureof ZOO-280 C.

In contradistinction to the naphthalene-based process, crude phthalicanhydride produced by the catalytic air oxidation of o-xylene containsno polymerizable components which must be eliminated by a polymerizationand separation pretreatment prior to the distillation step.

However, the crude product produced from o-xylene does contain, inaddition to phthalic anhydride, (PA) varying quantities of phthalicacid, benzoic acid, maleic acid, maleic anhydride, o-toluic acid,phthalide, higher boiling components, and a non-distillable residue.Because of economic and technical considerations, it is desirable toconvert the acids, before entrance into a distillation plant, into theanhydrides thereof, with water being split off, and to remove otherlow-boiling components entirely or in part.

It is conventional to conduct this dehydration step in such a mannerthat the entire crude product is heated in an agitated vessel or incascades of agitated vessels, to temperatures of between 190 and 280 C.This process can be conducted discontinuously or continuously, butrequires on a large industrial scale, considerable investment for thepretreatment plant. In addition, residence times of between about fiveand 100 hours are common, thereby increasing the investment capital evenfurther for on-stream inventory. This thermal treatment isconventionally conducted in the liquid phase without any priorseparatory treatment of said liquid phase.

SUMMARY OF THE INVENTION Bearing in mind the above-describeddisadvantages of prior art processes, an object of this invention is toprovide an improved preliminary purification process, hereinafter calledpretreatmentor thermal treatment, the latter term being found in priorart references.

A particular object is to markedly reduce the residence times associatedwith the old processes.

Another object is to provide a pretreatment wherein such mild reactionconditions exist that there are encountered only negligibledecarboxylation or cracking reactions.

Upon further study of the specification and claims, of the objects andadvantages of the present invention will become apparent.

To attain the above objects, the pretreatment is conducted by passingthe molten crude phthalic anhydride in the form of a film having athickness of at most two mm. over a heated surface and heating it duringthis procedure within at most 60 seconds to the treatment temperature.In this way the splitting of water from any maleic and phthalic acidpresent in the crude product, as well as the vaporization and removal ofany nascent water from the crude product is accomplished in a muchshorter time than previously.

DETAILED DISCUSSION OF THE INVENTION Whereas the film thickness must notexceed two mm., it is preferred to use even smaller thicknesses, lessthan 1.7 mm., and especially in the range of 0.6 to 1.5 mm. Similarly,while the pretreatment heating time must not exceed 60 seconds, it iseven more desirable for it to be shorter, no more than 40 seconds, andpreferably in the range of five to 30 seconds. By pretreatment heatingtime is meant the time at which the crude molten phthalic anhydride isat a temperature of 200280C. The pretreatment is normally conductedunder atmospheric pressure. However, a reduced pressure (e.g., in therange of 400-760 torr [mm. Hg] is likewise possible; in this connection,the film temperature must be below the boiling temperature of thephthalic anhydride at this lower pressure.

By operating according to this invention, it has been unexpectedlydiscovered that not only can the water be split off from the phthalicand maleic acid completely, but also, despite very short residencetimes, a large portion, e.g., 10 to 50 percent, of the impurities morevolatile then phthalic anhydride are vaporized. Consequently, crudephthalic anhydride subjected to the pretreatment of this invention canbe freed of the remaining volatile components by a less costlydistillation step than heretofore was normally employed.

The preferred technique of this invention comprises conducting themolten phthalic anhydride film over a heating surface subdivided into aplurality of separated heating zones, the temperature of each zoneincreasing in the direction of film flow. The film is thus heated withinnot more than one minute from, the example, 170 C. to about 280 C. Thethinness of the film permits a relatively rapid thermal equilibrium tooccur with respect to the film temperature on the heating zones. Inaddition, because of the large specific surface of the film, a high rateof mass transfer is achieved, whereby the water and other readilyvolatile components in the film enter into the gaseous phase.

A particularly beneficial technique is to provide that the film passesthe hottest heating zone, just below the boiling point of the phthalicanhydride for only a very brief period. For example, the film should bein contact with a heat exchange zone of about 275--280 C. for not morethan two, preferably not more than one seconds. In this way, evaporationlosses are substantially avoided, and no accumulation of undesireddeposits occurs on the hot heating surface.

Preferably, the molten phthalic anhydride film is allowed to pass overfour heating zones with temperatures of 220-230 C.; 240-245 C.; 255-265C. and 275 280 C.

The time-temperature conditions are preferably as follows:

Zone Temperature Residence Time I 220-230 C. lO-ZO sec.

2 240245 C. 48 see.

3 255-265 C. 2-4 sec.

4 275-280 C. 0.2l sec.

In the prior art pretreatment processes residence times in the rangefrom five to 100 hours are used. These processes prescribe the longpretreatment times irrespective of the type of the crude phthalicanhydride (i.e., obtained from naphthalene or o-xylene) and it was agenerally accepted opinion that pretreatment must last for these longtimes in order to convert the contaminants to products capable to beseparated by distillation. We have however found that this is only validfor the condensation of naphthoquinone present in crude phthalicanhydride obtained by oxidation of naphthalene to a nonvolatile resinbut not for the phthalic acid and maleic acid present in the crudephthalic anhydride obtained by o-xylene oxidation. These acids areconverted under pretreatment conditions to the respective anhydrides ina very short time. For shortening the pretreatment as a whole, time andpath of diffusion must also be reduced in order not to make diffusionthe rate controlling step. Thus our invention is based on a reduction ofthe diffusion path together with the knowledge about the fast formationof the anhydrides, which combination allows a considerable reduction ofthe pretreatment time for crude from o-xylene.

The thus-prepurified phthalic anhydride is collected in a tank fromwhich it is fed to a distillation plant.

In accordance with the preferred embodiment of the invention, the moltenphthalic anhydride is allowed to pass over the inner surface of avertical column generally having an inside diameter of at least mm. Thecrude molten product is distributed uniformly over the circumference ofthe column at the upper end of the column, for example, with the aid ofa rotor. It is also advantageous for the surface of the film of phthalicanhydride formed on the heating surface to be constantly renewed byhaving mechanical components such as vanes move through the film atright angles to the direction of film flow. When conducting thepretreatment in a vertical pipe, the latter is equipped with anagitator, the blades of which rotate at a distance from the heatingsurface corresponding to the thickness of the film. Thereby, there isachieved a thorough mixing of the phthalic anhydride film resulting in asatisfactory heat transfer from the heating surface to the film.

The various heating zones of the pretreatment apparatus can bemaintained at the desired temperatures either electrically or by a heattransfer fluid. In this connection, the heating surface exhibits thelowest temperature at the feed end for the crude phthalic anhydride, andthe highest temperature at the discharge end, thereby simulatingindirect countercurrent heat exchange. The pretreatment can also beconducted at a uniform temperature; in this case, the process isconducted preferably with smaller film thicknesses, e.g., 0.5 to 1.2mm., the lower the pretreatment temperature employed, the smaller thethickness.

The film thickness can be adjusted in a simple manner by the spacing ofthe agitator blades from the heating surface. The residence time at thepretreatment temperature is dependent on the flow rate of the film andthe length of the heating surface. The residence time at thepretreatment temperature is dependent on the flow rate of the film andthe length of the heating surface. This residence time can be adapted tothe desired degree of purity by varying the length of the heatingsurface, e.g., by positioning several film heating elements in series.

BRIEF DESCRIPTION OF DRAWING The attached drawing is a schematicrepresentation of a preferred embodiment of the invention.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The following preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

EXAMPLES EXAMPLE 1 This example will be explained with reference to theattached drawing.

Crude phthalic anhydride, produced by the vapor phase oxidation ofo-xylene is collected in tank 1, and, is passed in the liquid phase tothe upper end ofa a vertical cylindrical column 2 along thecircumference thereof. The formed film thereafter flows by gravity downthe column 2 having an internal diameter of 82 mm., a height of 500 mm.,and a surface area of 0.129 m It is divided into four separatelycontrollable heating zones 3, 4, 5, 6 by means of externally mountedelectrical strip heaters ofa width of 120 mm. The temperature of thecolumn wall was set, with the aid of the heating strips, so that therewere measured in the uppermost zone 3, a wall temperature of 220 C.; inthe second zone 4, of 240245 C.; in the third zone 5, of 255-265 C.; andin the lowermost zone 6, a temperature of 275280 C. The column wasplaced in communication with the atmosphere at the upper end, so thatthe preliminary purification was conducted under atmospheric pressure,and the more volatile components escaping from the film could freelyemanate from the column.

At the upper end ofthe column 2, 25 kg./h. of molten crude phthalicanhydride was charged at a temperature of 170 C. This crude productconsisted of 94.4 percent by weight of phthalic anhydride, 4.5 percentby weight of phthalic acid, 0.4 percent by weight of maleic acidanhydride and maleic acid, 0.1 percent by weight of benzoic acid, 0.1percent by weight of phthalide, 0.1 percent by weight of o-toluic acid,0.18 percent by weight of residue and unknown components having a higherboiling point than phthalic anhydride, and 0.2 percent by weight ofunknown components having a boiling point lower than phthalic anhydride.The water-insoluble residue amounted to 0.13 percent by weight. The hotmelt exiting from the column 2 was passed, via a conduit 7 into acollecting tank 9. The residence time in the apparatus was about nineseconds. The thickness of the film was approximately 0.5 mm. The productwas obtained at a rate of 24.76 kg./h. and contained 99.3 percent byweight of phthalic anhydride 0.2 percent by weight of maleic acidanhydride, 0.06 percent by weight of benzoic acid, 0.08 percent byweight of o-toluic acid, 0.1 percent by weight of phthalide, 0.19percent by weight of residue and unknown components having a higherboiling point than phthalic anhydride and 0.07 percent by weight ofunknown components having a lower boiling point than phthalic anhydride.The concentration of water-insoluble residue had risen to 0.15 percentby weight. The phthalic anhydride subjected to the pretreatmentaccording to the invention was then pumped from tank 9 into thedistillation column 10 in which a high boiling residue was separated at11 and the purified phthalic anhydride was distilled overhead at 12.

EXAMPLE 2 The pretreatment of this invention can also be conducted onvertically disposed plates, the surfaces of which are corrugated and areheated from the inside. Especially suitable is an inclined pipe having aplanar underside. The underside of the pipe forms an inclined planewhich is heated from underneath.

The phthalic anhydride is charged at the upper end of the pipedistributed over the entire width of the planar underside, and flowsdownward along this side in the form of a film, and is heated toprogressively higher temperatures. In the pretreatment of thisinvention, it is to be understood that the hating in the pretreatmentstep not only converts the phthalic acid and maleic acid impurities intothe corresponding anhydrides, but also because of the temperaturesinvolved, inherently evaporates low boiling impurities, e.g., the watersplit off during the formation of the anhydrides. Conversely, though, itis not desired to vaporize the phthalic anhydride during thispretreatment step.

The preceding two examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexample.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

We claim:

1. In a process for the continuous purification of crude phthalicanhydride produced from o-xylene, said process comprising subjectingsaid crude phthalic anhydride containing maleic and phthalic acidimpurities to a thermal treatment in the liquid phase without any priorseparatory treatment of said liquid phase, and then subjecting resultantthermally treated phthalic anhydride to distillation, the improvementwherein said thermal treatment comprises passing a thin film of saidcrude phthalic anhydride in the molten phase in contact heat exchangerelationship with a heated surface to heat said crude phthalic anhydrideto ZOO-280 C. but less than the boiling temperature of phthalicanhydride and under an absolute pressure of about 400-760 mm. Hg, saidthin film having a thickness of not more than two mm., and said heatexchange being conducted for not more than 60 seconds, whereby themaleic and phthalic acid impurities are converted to the correspondinganhydrides, and impurities more volatile than phthalic anhydride arevaporized and removed.

2. A process according to claim 1 wherein said heating surface comprisesthe inner surface ofa vertical column.

3. A process according to claim 1, further comprising the step ofpassing agitator blades through the moving at right angles to thedirection of flow of the film to continuously obtain fresh filmsurfaces, thereby increasing the rates of heat exchange and masstransfer.

4. A process according to claim 1 wherein said heat exchange isconducted for not more than 40 seconds.

5. A process as defined by claim 1 wherein the filmthickness is lessthan 1.7 mm.

6. A process as defined by claim 1 wherein the film thickness is 0.6 to1.5 mm.

7. A process according to claim 1 wherein said heat exchange isconducted for 5-30 seconds.

8. A process as defined by claim 7 wherein the film thickness is 0.6 to1.5 mm.

9. A process as defined according to claim 1, said heating surface beingsubdivided into a plurality of separate heating zones of differenttemperatures, the temperature of the zones increasing in the directionof the flow of the film.

10. A process according to claim 9 wherein said plurality of heatingzones comprises four heating zones having heating surface temperaturesof 220-230 C.; 240245 C.; 255-265 C.; and 275-280" C. respectively.

11. A process according to claim 10 wherein said heating surfacecomprises the inner surface of a vertical column.

12. A process according to claim 11, further comprising the step ofpassing agitator blades through the moving film at right angles to thedirection of flow of the film to continuously obtain fresh filmsurfaces, thereby increasing the rates of heat exchange and masstransfer.

13. A process as defined by claim 10 wherein the heat exchange at 275280C. is conducted for less than two seconds.

14. A process as defined by claim 10 wherein the heat exchange at275-280 C. is conducted for less than one second.

15. A process as defined by claim 10 wherein the heat exchange time at220-230 C. is 10-20 seconds; at 240-245" C., four to eight seconds; at255265 C., two to four seconds; and at 275-280 C., 0.2-1 second.

16. A process as defined by claim 15 wherein the film thickness is0.6-1.5 mm.

17. In a process for the continuous purification of crude phthalicanhydride produced from o-xylene, said process comprising subjectingsaid crude phthalic anhydride containing maleic and phthalic anhydrideimpurities to a thermal treatment, and then subjecting resultantthermally treated phthalic anhydride to distillation, the improvementwherein said thermal treatment comprises passing a thin film of saidcrude phthalic anhydride in the molten phase at an absolute pressure of400-760 mm. Hg in contact heat exchange relationship with a heatingsurface subdivided into four heating zones having heating surfacetemperatures of 220-230 C.; 240-245 C.; 255-265 C.; and 275-280 C., thetemperature of the zones increasing in the direction of flow of the filmand being less than the boiling temperature of phthalic anhydride, saidthin film having a thickness of not more than two mm., and said heatexchange being conducted for not more than 60 seconds, whereby themaleic and phthalic anhydride impurities are converted to thecorresponding anhydrides, and irnpurities more volatile than phthalicanhydride are vaporized and removed.

T D STATES PATENT FFKIE QETEWCATE Ul @RREQTWN Patent No. Q 3 65o DatedMarch 1972 Inventor(s) HUBERT GEHRKEN ET AL.

It is certified that error appears in the above-identified patent andthat said Letters Patent arehereby corrected as shown below:

CLAIM 3, Column 4, line 72: After "moving" insert film Signed and sealedthis 26th vday of June 1973.

( S.EAL) Attestr EDWARD M.FLETCHER,JR., v ROBERT GOTTSCHALK AttestingOfficer Commissioner of Patents FORM PC4050 (m'sg) uscoMM-oc some-P69 vQ U.$. GOVERNMENT PRINTING OFFICE: 1969 0-356-334,

2. A process according to claim 1 wherein said heating surface comprisesthe inner surface of a vertical column.
 3. A process according to claim1, further comprising the step of passing agitator blades through themoving at right angles to the direction of flow of the film tocontinuously obtain fresh film surfaces, thereby increasing the rates ofheat exchange and mass transfer.
 4. A process according to claim 1wherein said heat exchange is conducted for not more than 40 seconds. 5.A process as defined by claim 1 wherein the film thickness is less than1.7 mm.
 6. A process as defined by claim 1 wherein the film thickness is0.6 to 1.5 mm.
 7. A process according to claim 1 wherein said heatexchange is conducted for 5-30 seconds.
 8. A process as defined by claim7 wherein the film thickness is 0.6 to 1.5 mm.
 9. A process as definedaccording to claim 1, said heating surface being subdivided into aplurality of separate heating zones of different temperatures, thetemperature of the zones increasing in the direction of the flow of thefilm.
 10. A process according to claim 9 wherein said plurality ofheating zones comprises four heating zones having heating surfacetemperatures of 220-230* C.; 240-245* C.; 255-265* C.; and 275-280* C.respectively.
 11. A process according to claim 10 wherein said heatingsurface comprises the inner surface of a vertical column.
 12. A processaccording to claim 11, further comprising the step of passing agitatorblades through the moving film at right angles to the direction of flowof the film to continuously obtain fresh film surfaces, therebyincreasing the rates of heat exchange and mass transfer.
 13. A processas defined by claim 10 wherein the heat exchange at 275-280C. isconducted for less than two seconds.
 14. A process as defined by claim10 wherein the heat exchange at 275-280* C. is conducted for less thanone second.
 15. A process as defined by claim 10 wherein the heatexchange time at 220-230* C. is 10-20 seconds; at 240-245* C., four toeight seconds; at 255-265* C., two to four seconds; and at 275-280* C.,0.2-1 second.
 16. A process as defined by claim 15 wherein the filmthickness is 0.6- 1.5 mm.
 17. In a process for the continuouspurification of crude phthalic anhydride produced from o-xylene, saidprocess comprising subjecting said crude phthalic anhydride containingmaleic and phthalic anhydride impurities to a thermal treatment, andthen subjecting resultant thermally treated phthalic anhydride Todistillation, the improvement wherein said thermal treatment comprisespassing a thin film of said crude phthalic anhydride in the molten phaseat an absolute pressure of 400-760 mm. Hg in contact heat exchangerelationship with a heating surface subdivided into four heating zoneshaving heating surface temperatures of 220-230C.; 240-245* C.; 255-265*C.; and 275-280* C., the temperature of the zones increasing in thedirection of flow of the film and being less than the boilingtemperature of phthalic anhydride, said thin film having a thickness ofnot more than two mm., and said heat exchange being conducted for notmore than 60 seconds, whereby the maleic and phthalic anhydrideimpurities are converted to the corresponding anhydrides, and impuritiesmore volatile than phthalic anhydride are vaporized and removed.